Abstract
A current sensitive preamplifier designed for low-temperature scanning tunneling microscopy applications is presented. It combines the dc current measurement necessary for the feedback loop operation with a low noise ac measurement used for spectroscopy. The active device is a high electron mobility transistor which was chosen for its low input capacitance and excellent low-temperature performance. The power dissipation of the transistor can be kept at about 10 μW making it compatible with a variety of cryogenic systems. The ac current sensitivity is about 4 fA/√Hz at 4.2 K.
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